How Java Virtual Machine can work on system without virtualization support?

If hardware support is a must for virtualization, how can Java Virtual Machines run on machines without support for virtualization ? Or is JVM not开发者_如何学C a virtual machine ?

A JVM is not virtual in the same sense as a VirtualBox or VMWare virtual machine. It is a 'machine' that implements the Java bytecode, not a virtualized version of actual hardware.

The term-of-art 'virtual machine' was coined a very long time ago for the following scenario:

1. make up a computer, like Knuth's MIX.
2. write a computer program that implements the made-up computer.
3. run programs

When this virtual machine runs, it's a completely ordinary program, running completely in user mode. It needs no special help from the hardware or operating system to work reasonably well. This is especially true of the JVM, since the Java byte code does not deal with low-level hardware I/O or other things which are hard to simulate.

Later, historically, (to pick a particular instance), IBM invented VM/370. VM/370 uses the other sense of the term 'virtual machine'. In this later sense, the hardware and operating system cooperate to allow a single physical machine to host multiple virtual instances of (more or less) the same architecture, in which multiple copies of the whole operating system are written as if they are running on more or less bare hardware. Later, the X86 was designed with features to facilitate this.

So, yes, any virtual machine is making use of some physical hardware, unless you implement it with pieces of paper passed around a table (pace John Searle). But when the virtual machine bears no resemblance to the machine it is running on, then there's no need for special help from the operating system and hardware, and no need for anything as complex as VM/370, or VMware.

If hardware support is a must for virtualization, ...

Let me stop you right there :-)

There is a difference in concept between the JVM (software virtualization) and (for example) a VMWare VM (hardware-assisted virtualization).

The JVM (and other software-based VMMs such as the ones that allow to to emulate x86 on Solaris hardware - I think Bochs and possibly DosBox fall into this category) runs like any other application, using the operating system to gain access to the hardware, or emulating its own hardware purely in software.

VMWare, and the other VMMs optimised for speed, rely on hardware support. In other words, they run on the hardware as if they have full access to the hardware and, only when they try to do something they're not supposed to does the OS captures that attempt and fake it.

That's why VMWare runs so much faster than the software-only emulators. It's because, for the vast majority of the time, it's actually running on the real hardware.

The JVM is a virtual machine, but it doesn't require any additional support from the Operating System. Instead of virtualising instructions for a particular CPU it executes java bytecode.

The JVM is a virtual machine for running Java, in other words it emulates a machine which would be capable of running java. It is a confusing choice of names, but it comes from the general meaning of "machine" not from the more common Virtual Machine meaning.

The JVM, like a regular VM emulates the execution of instructions, but in the case of the JVM the instructions being emulated are Java Instructions, and in the case of a VM they are Hardware Instructions as would be executed by an OS running on the same hardware.

Yes the JVM does access hardware, however this is why you install a MAC or WINDOWS JVM since the instructions are translated by the JVM and acted upon depending on the installation of the JVM, for example, open file dialog on mac opens the mac dialog and windows JVM opens the windows dialog.

So its not being virtualized by the system, but the bytecode is being virtualized by the JVM you installed. It's basically like an application that reads something(bytecode) and does something(access hardware, or other stuff).

It should be noted that nothing stipulates that a JVM does not (have to) have HW virtualization access. There are notable exceptions, but to which the answered poster alluded, few CPs exist that run Java bytecode natively. Maybe someday a Java bytecode HAL or TIMI will be commonplace to put the JVM into the same class as the formalized HW virtualization?